Reactive oxygen species (ROS) are produced as a by-product of cellular metabolism and through exposure to ultraviolet and ionizing radiation and environmental carcinogens. A major base damage produced by ROS is 7,8-dihydro-8-oxoguanine (8-oxoG). Unlike normal guanine, 8-oxoG has the propensity to mispair with adenine during DNA replication, resulting in the fixation of G:C to T:A transversion mutations. Oxidatively modified bases, such as 8-oxoG, are repaired primarily by the base excision repair pathway (BER), the first steps of which are the recognition and excision of the damaged base by a specific DNA glycosylase. The major mammalian enzyme for removing 8-oxoG from DNA is 8-oxoguanine-DNA glycosylase (OGG1). OGG1 is a bifunctional enzyme, having both 8-oxoG excision activity and a weak AP-lyase strand incision activity at abasic sites. Following excision of 8-oxoG by OGG1, the resultant abasic site is further processed in sequential steps by several enzymes to complete repair.[unreadable] Studies of OGG1 knockout mice and immunodepletion experiments suggest that OGG1 is the major mammalian 8-oxoguanine repair activity in non-transcribed DNA. It is widely accepted that accumulation of oxidative DNA damage over time can lead to cancer. A role for OGG1 in tumor suppression is suggested by the frequent loss of the OGG1 chromosomal locus in human lung and renal cancers and by significantly lower OGG1 activity among lung cancer patients compared to controls. Increased late-onset lung tumors in knockout mice deficient in repair of 8-oxoG, elevated 8-oxoG levels in lung tissue of lung cancer patients and decreased repair of 8-oxoG demonstrated in several human cancer cells lines suggest that cancer and 8-oxoG repair capacity may be linked. Changes in the OGG1 coding sequence that result in amino acid substitutions that affect function, abundance, or intracellular location could be anticipated to impact genomic 8-oxoG levels, and thereby influence genomic stability and carcinogenesis.[unreadable] Several OGG1 polymorphisms have been reported and positively correlate with a variety of cancers. A frequently occurring polymorphism results in the substitution of serine for cysteine at position 326 in the C-terminus of OGG1. The allele frequency of S326C OGG1 measured in human populations ranges from 0.13 to as high as 0.62 and varies significantly with ethnicity . Association studies have identified that individuals homozygous for the S326C OGG1 allele have increased incidence of lung, prostate, and orolaryngeal cancers. A previous study found decreased catalytic efficiency (kcat/Km) of purified polymorphic S326C OGG1 , while another study implicated the S326C genotype with decreased 8-oxoguanine repair capacity in vivo. We characterized the glycosylase and AP-lyase activities and DNA damage binding affinity of purified S326C and found novel functional defects in the polymorphic OGG1 and a distinct dimeric DNA binding conformation compared to the wild-type enzyme. Our results confirm that S326C has decreased repair activity towards 8-oxoG paired with C and further show that S326C OGG1 is particularly deficient in 8-oxoguanine excision activity when the lesion is opposite T or G. The stimulation of wild-type OGG1 by APE1 results in increased rates of 8-oxoG excision, and is believed to be an important step in the regulation and coordination of base excision repair in vivo. We show that S326C OGG1 is not significantly stimulated by APE1, unlike the wild-type enzyme, thereby the coordination of BER may be perturbed during repair of 8-oxoguanine by S326C OGG1. We observed decreased repair activity and dimeric conformation of S326C OGG1 expressed in human cells, thus the altered activity and dimeric stoichiometry of the S326C OGG1 variant may be relevant in vivo. We characterized the enzymatic activity of R229Q and determined the effect of R229Q expression on KG-1 survival following exposure to DNA damaging agents. Our results show that R229Q OGG1 is highly thermolabile and rapidly inactivated at physiological temperatures both in vitro and in vivo. Expression of both nuclear and mitochondrial R229Q OGG1 sensitized KG-1 cells to killing via an apoptotic pathway following exposure to menadione and 8-oxodG, thus R229Q promotes apoptosis following ROS and oxidized nucleoside exposure. Initially reported as a unique somatic mutation in KG-1 cells, we report that the R229Q allele is a documented polymorphism in human populations. With the significant incidence of the allele in the population, our observations of OGG1 structural destabilization and increased cell killing following induction of oxidative DNA damage resulting from the R229Q polymorphism suggest that the variant may be a potential marker for cancer susceptibility. These results suggest that decreased 8-oxoguanine repair in KG-1 is due to thermolability of R229Q OGG1 and that the enzyme variant increases cellular susceptibility to killing resulting from oxidative DNA damage. The R229Q OGG1 variant is a validated polymorphism prevalent in world populations and not an isolated mutation in KG-1 cells, thus the R229Q OGG1 allele may be a novel marker for cancer susceptibility.